1. Field of the Invention
The present invention relates to a sensorless brushless motor suitable for application to various types of small motors.
2. Description of Related Art
Generally, as a small motor, there is proposed, for example, a three-phase sensorless brushless motor which has no sensor for detecting a rotational position of a rotor and no brush. In such a motor, a rotational position of the rotor is detected not by using a sensor such as a Hall element but by using a back electromotive force (back EMF) to be induced in excitation coils U, V and W constituting a three-phase stator when the rotor is rotated, then by determining a respective timing for energizing respective excitation coils U, V and W based thereon and causing to flow a current therethrough accordingly, the rotor is rotated. Therefore eliminating the need for an excitation sensitivity element such as a position detecting sensor constituted by a Hall element and the like.
As an example of such sensorless brushless motors, there has been proposed a three-phase outer rotor type sensorless brushless motor as shown in FIGS. 4, 5 and 6. A rotor 1 of this three-phase outer rotor type sensorless brushless motor is formed of a cylindrical member constituted by two North poles and two South poles, which are arranged alternately in a ring form as shown in FIG. 5. Further, a stator 2 is disposed within the cylindrical rotor 1, has three-phase excitation coils U, V and W each wound around an iron core opposing the rotor 1, and spaced apart from each other at an electrical degree of 120° as shown in FIG. 5.
These three-phase excitation coils U, V and W are connected in Y-connection as shown in FIG. 4. An excitation coil energizing circuit 3 causes an excitation current to flow, for example, from the excitation coil U to V, and from the excitation coil U to W, then sequentially switches to flow from the excitation coil V to W, V to U, W to U and W to V, repeating these sequential switching therebetween.
In this arrangement, one of the excitation coils is selected by a back EMF detection excitation coil selecting circuit 4 as an excitation coil through which no excitation current is flowed, for example, in the case that an excitation current flows from excitation coil U to V, the excitation coil W will be selected as the one. A back EMF induced in this excitation coil W, through which no excitation current flows, due to the rotation of the rotor 1 is supplied to one of input terminals of a voltage comparator circuit 5 constituted by an operating amplifier circuit, and at the same time, a neutral point voltage obtained at a neutral point in the connections of the exciting coils U, V and W is supplied to the other one of the input terminals of the voltage comparator circuit 5 as a reference voltage.
At an output terminal of the voltage comparator circuit 5, there is obtained a rectangular wave signal corresponding to a back EMF for use of detection of a rotational position of the rotor 1, which was induced in the excitation coil flowing no excitation current, and this rectangular wave signal corresponding to the back EMF for use of the position detection of the rotor 1 is supplied to an energizing timing signal generating circuit 6 and to a back EMF detection excitation coil select timing signal generating circuit 7.
The energizing timing signal generating circuit 6 receives a rectangular wave signal for detection of the position of the rotor 1, and generates energizing timing signals US, VS and WS for selecting respective excitation coils U, V and W to be energized sequentially, each having a phase difference of 120° as shown in FIGS. 6A, 6B and 6C. These energizing timing signals US, VS and WS are supplied to an excitation coil energizing circuit 3, and whereby respective two-phase excitation coils to flow an excitation current therethrough are sequentially switched over. Also, in the energizing timing signal generating circuit 6, an FG signal for detecting a rotational speed of the rotor 1 is generated. A numeric 6a depicts an output terminal of this FG signal.
Further, in response to the rectangular wave signal for detection of the position of the rotor 1 outputted from the voltage comparator circuit 5, the back EMF detection excitation coil select timing signal generating circuit 7 generates a select timing signal for selecting an excitation coil, through which no excitation current flows, for use of detecting the back EMF. The select timing signal obtained in the back EMF detection excitation coil select timing signal generating circuit 7 is supplied to the back EMF detection excitation coil selecting circuit 4, whereby an excitation coil for detecting the back EMF is selected.
Further, in FIG. 4, numeric 8 depicts a PWM (pulse-width modulation) circuit for controlling a rotational speed of the rotor 1 in the sensorless brushless motor. This PWM circuit 8 generates a pulse width modulation signal including pulse widths corresponding to an error in the rotational speed as shown in FIG. 6D. The pulse width modulation signal including the pulse widths corresponding to the error in the rotational speed generated in the PWM circuit 8 is supplied to the excitation coil energizing circuit 3. Numeric 8a depicts an error signal input terminal to which the error signal of the rotational speed of the rotor 1 is supplied.
In the excitation coil energizing circuit 3, respective excitation coils U, V and W are caused to be sequentially energized on the basis of a logical product between respective energizing timing signals US, VS, WS as shown in FIGS. 6A, 6B, 6C and a pulse width modulation signal as shown in FIG. 6D so as to ensure a predetermined rotational speed to be obtained.
In this sensorless brushless motor, a rotational position of the rotor 1 is detected not by means of a position sensor such as a Hall element but by detecting a back EMF to be induced in the excitation coils U, V and W when the rotor 1 is rotated. For that reason, if any kind of noise is superimposed on this back EMF, the accuracy in detection of the position of the rotor 1 will deteriorate.
In particular, at the time of a low speed rotation of the motor, because of this back EMF itself being very small, its detection accuracy will be further deteriorated. Although various kinds of noises may be considered as a noise to be superimposed on this back EMF, when the rotor 1 is operating by using the pulse width modulation signal as described above, an influence of a steep voltage change in the pulse width modulation signal on an excitation coil being energized becomes remarkable.
The noise generation in the excitation coil due to the steep voltage change will be described more in detail. The excitation coils being energized are intermittently applied with an external voltage via the pulse width modulation signal, as a result, the excitation coil for detecting the back EMF which is connected in common to the other excitation coils being energized is also subjected to a voltage change.
The steep voltage change occurring in the excitation coil for use of detecting the back EMF is generated such that at first a voltage change appears at its terminal on the common side, then this voltage change propagates to its another terminal on the other side, thereby an instantaneous voltage is generated at the both terminals of the excitation coil with a time difference of propagation, which becomes a noise to be superimposed on the back EMF.
Therefore, if the pulse width modulation signal given is as shown in FIG. 7A, a back EMF 4a obtained at the output terminal of the back EMF detecting excitation coil selecting circuit 4 will become as shown in FIG. 7B, in which a voltage change due to a leading edge and/or a trailing edge of the pulse width modulation signal 8b will be superimposed.
In the case of an exemplary embodiment shown in FIG. 4, a reference voltage of the voltage comparator circuit 5 is zero voltage as shown in FIG. 7B, and an output signal obtained at the output side of the voltage comparator circuit 5 is fluctuated as shown in FIG. 7C, thereby there arises a problem that the position of the rotor 1 cannot be detected accurately.
Therefore, a method as disclosed in Patent document 1 is conventionally proposed in which an output signal from the voltage comparator circuit 5 as shown in FIG. 7C is sampled at a timing which is shifted from the leading edge or the trailing edge of the pulse width modulation signal 8b by approximately one-half of a cycle of the pulse width modulation signal, and a point at which a sampled signal becomes high-level “1” or low-level “0” is specified as a position detecting point. (Patent Document 1: Japan Patent-Application Publication No. H11-4595.